The invention relates to the field of automated optical surface inspection and more particularly to optical systems for inspection of planar surfaces.
The density of information stored on magnetic disks used in disk drives requires that the surfaces of the disk be formed with extreme precision and that the inspection processes identify defects of microscopic size. In addition to optical inspection other tests typically include glide tests and selective magnetic tests. The quantity of magnetic information which can be stored on a disk is too large to allow cost effective testing of all of the magnetic bit locations and, therefore, only a small subset of surface is typically magnetically tested. Optical inspection requires less time than magnetic testing and is more cost effective for large scale manufacturing. It is also sometimes desirable for a test stand to be used which is capable of performing different kinds of tests by utilizing multiple heads which perform the individual tests. Compact test head design in such a multitest system is an important consideration. The ability to test or inspect both planar surfaces of the disk is also desirable.
Prior art systems include the commonly assigned U.S. Pat. No. 5,898,492 which uses reflected light from the disk surface and captures scattered light from piano mirrors positioned at an angle above the surface. The described system uses a telecentric lens assembly and a rotating polygon scanner to direct both the incident light and combined reflected and scattered light. The system is suitable for simultaneous inspection of both surfaces of a disk. In this system the disk is not rotating while being inspected rather is moved linearly through the scanning area while the inspection apparatus remains in a fixed position. This design is poorly suited to the multihead tester application where the space constraints are more stringent.
The surface being inspected in a preferred embodiment is a highly reflective thin film magnetic disk for use in a disk drive. The invention uses laser light directed at an off-axis parabolic mirror which focuses the beam on the disk surface and also serves as the collector for scattered and specular light returned from the surface. The selection of the parabolic mirror allows the system to be made extremely compact for use in a multitester application and allows inspection of both sides of a disk simultaneously by positioning a parabolic mirror above and below the disk. The parabolic mirror is oriented with its axis parallel to a radial line on the disk surface. Specular and scattered light returned from the surface onto the parabolic mirror is divided into appropriate fields and directed onto detectors. Two or more piano mirrors can conveniently be used to separate the light into fields by placing the mirrors in series by using concentric apertures in the mirrors. In the preferred embodiment a polarized laser is used in conjunction with a polarizing beam splitter and a quarter-wave plate to route the reflected beam to a detector while allowing the original beam to be directed through the same optics.
When used in a multi-station test tool, preferably the parabolic mirror and selected additional components are commonly mounted on a translatable stage which is moved along a radius of the disk when the optical inspection is being performed. Other components of the system, the laser, for example, can remain in a fixed position.
In an embodiment of the invention the disk is mounted on a spindle that provides rotatable support and which is connected to a motor for rotating the spindle and the disk. The system of the invention can be used to inspect one or both planar surfaces of the disk by providing duplication of selected components appropriately oriented with respect to the second surface. The system includes at least a first parabolic mirror that is radially positionable over the planar surface to capture specular light and at least a portion of scattered light from the portion of the planar surface to be inspected. The laser beam is conditioned and directed by optical components onto the parabolic mirror and then focused onto the planar surface. In the preferred orientation, the parabolic mirror captures a portion of scattered light and specular light coming from the surface. The light captured by the parabolic mirror is then separated into at least two fields: one containing the specular light and one or more fields of scattered light. These fields of light are directed to appropriate detectors.